Traditionally, measurements are taken visually using hand tools or optical comparators. However, these tools are time-consuming and have limited accuracy. On the other hand, a coordinate measuring machine (CMM) uses coordinate processing techniques to measure the height, width, and depth of a part. In addition, such machines can automatically measure targets, record data, and obtain GD&T measurements. How do use coordinate measuring machines, how are they constructed and what are the advantages? We tell you about it below!
What is a coordinate measuring machine (CMM)?
A coordinate measuring machine (CMM) is a contact model using a touch probe, a spherical object to measure, or a non-contact model using other methods such as cameras and lasers. Some models for the automotive industry can even measure objects larger than 10 m.
The advantages of a coordinate measuring machine (CMM)
One of the advantages of a coordinate measuring machine (CMM) is that it can measure high-precision features that are difficult to measure with other measuring machines. For example, it is difficult to measure the 3D coordinates of a specific point (a hole, etc.) from a virtual starting point using hand tools such as calipers or micrometers. In addition, measurements using virtual points and virtual lines, and geometric tolerances are difficult to perform on other measuring machines, but they can be performed on 3D CMMs.
Design of a coordinate measuring machine (CMM)
The following is the design of a coordinate measuring machine (CMM):
Typically, most CMMs are bridge or gantry machines. Apply a spherical contact point attached to the probe tip to the object on the platform, and determine and measure the coordinate values in three dimensions (X, Y, Z). The controller is mainly used for 3D measurement of molds such as automotive parts and various mechanical parts, 3D measurement of objects such as prototypes, and measurement of differences with drawings.
Coordinate measuring machine (CMM) accessories
The coordinate measuring machine has the following accessories:
- Stylus / probe tip – The stylus of a contact type CMM has a spherical diameter. The probe tip is often made of hard materials such as zirconium or ruby.
- Granite table – To ensure high precision measurement, the CMM surface is usually a slab made of stone. The stone top plate changes very little over time and is not easy to scratch, so it has the advantage of providing long-term and stable use.
- Fittings – One of the most important tools in CMM operation is the fixture used to hold the measuring disc in the correct position. If the measuring unit is clamped, it will not move while the CMM is running, as the movement of the part can cause errors. Common tools are mounting plates, clips, and magnets.
- Compressors and air dryers – Mechanical CMMs require an air compressor with a dryer. These can be found on a standard bridge or gantry CMMs.
Coordinate measuring machine (CMM) software
There are currently two types of software for coordinating measuring machines.
- Software for our CMMs that we have developed independently for each CMM manufacturer.
- Software developed by a third party that can be used by multiple manufacturers’ measuring machines.
How to use a coordinate measuring machine (CMM)
Allow the measuring target to accustom to room temperature (usually 68 °F) in the metrology lab for at least 5 hours before taking the measurement. This will avoid measurement errors and discrepancies due to thermal expansion. Measurements are made manually by pointing the probe to the desired measurement location or using the control computer. The CMM will record the X, Y, Z coordinates of the probe position. As points continue to be taken, the system software will calculate specific dimensions such as:
- angle and other critical dimensions.
Calibrating the stylus (probe calibration)
To begin measuring accurately, the needle (probe tip) that contacts the object must be calibrated for two reasons:
- The first is to determine the coordinates of the center of the ball stylus.
- The second is to set the diameter of the ball of the needle.
By setting the diameter, you can calculate it by moving the radius from the point of actual contact (outside the ball) to the ball center coordinates.
Precautions when using a CMM
While some models can measure within 0.1 μm, proper use and handling are critical to measurement accuracy. Check that moving parts move horizontally and vertically during operation. You should also use a measurement standard or similar to check the reading errors. To obtain accurate measurements, set the target temperature in the metrology laboratory to room temperature. Alternatively, the measurement parameters must be set to correct for temperature differences. For touch probes, the contact speed between the probe and the object must be constant during the measurement.
CMM maintenance and calibration
Ordinary CMMs require regular maintenance and inspection to continuously perform high-precision measurements. Especially for bridge CMMs with mechanically driven sliding parts, regular replacement of worn parts, lubrication, and cleaning of the system are required for optimal performance.
Measurements on a coordinate measuring machine (CMM)
Coordinate measuring machines cmm usually have a device coordinate system set up on the object. The device coordinate system is defined by the device, for example, the axis moves laterally as the X-axis and the direction perpendicular to the tabletop is the Z-axis. Since physical positioning in machine coordinates is difficult and inaccurate, the work coordinate system is aligned with the reference plane or datum line of the workpiece. This way of aligning the orientation of the workpiece with the orientation of the reference coordinates is called alignment.
How do I set coordinates?
Setting a working coordinate system requires three pieces of information:
- The first is the reference plane, and the direction perpendicular to this plane is the Z-axis.
- The second is the reference line, usually the X-axis and the Y-axis vertically. A straight line can be measured directly from an object, or it can be a straight line connecting two different points (such as two holes). dashed line.
- The third point is the origin. This origin is point 0 for each X, Y and Z coordinate value. You can also specify a specific point (for example, the center of a specific hole) as the origin or a virtual point (intersection) where two lines intersect.
Measuring dimensions and 3D features
Typically, the user selects the measurement target using the software menu and begins the measurement. In the case of a contact coordinate measuring machine, the tip of the needle touches the object to be measured and determines the measuring point. Objects are measured by measuring the minimum number of measurement points specified for each object. If the number of measurement points is greater, it is usually calculated using the least-squares method. In addition to planes, measuring elements include:
- cones and spheres.
- Measure 3D size and shape by calculating distances and angles between measured elements.
Virtual figures (projection)
Some elements have 3D shapes, such as cylinders and cones, while others do not have 3D shapes, such as lines and circles. These elements are usually projected onto a plane (moved perpendicular to the plane) so that they can be measured correctly. The projected plane is called the reference plane or projection plane.
Measuring virtual figures
CMMs can also measure using virtual lines and points. Various examples of virtual figures are used, such as intersections of lines, tolerances of planes, intersections between planes, and circles between cones and planes. It can be said that measuring with these virtual features, which are difficult to measure with hand tools such as calipers, is unique to 3D measurement.
Problems with coordinate measuring machines (CMMs)
Like all equipment, CMMs can have their advantages and disadvantages, which we discuss below:
- Measurement stability – Proper setup and measurement require specialized knowledge and skills. Maintaining the right temperature in the measurement cell to stabilize the temperature of the object is required.
- Responsiveness – It is difficult to handle frequent product changes due to the need to calibrate each time a different probe setting and angle are changed. Due to the need for a measurement chamber, it is difficult to take frequent measurements when manipulating objects.
- Cost and effort – Installation requires a lot of space, and a quality control lab has to be built, which is extremely expensive. The cost of maintaining the measurement environment and measurement equipment can be prohibitive. CMM programming takes a lot of time for many reasons. The time it takes to send the part to the quality lab, get the part at the right temperature, fix it, calibrate each probe tip, and make the measurement.
Optical Coordinate Measuring Machines
Optical CMMs are portable proximity devices. These CMMs use an armless system with an optical triangulation method to scan and measure objects in 3D. With advanced image processing technology, the optical CMM is very fast and guarantees metrology-level accuracy. Optical triangulated scanners are particularly beneficial for the development of Industry 4.0. Although optical CMMs are slightly less accurate, they are still used in a wide range of applications. Optical CMMs are used in conjunction with conventional CMMs to eliminate manufacturing bottlenecks. Therefore, parts that require a significant level of accuracy are checked using conventional CMMs. All other components can be evaluated with a more economical optical CMM that provides satisfactory
- flexibility and speed.
With CMMs, we can accurately use coordinate processing techniques to measure the height, width, and depth of parts. This is extremely important because this action will make our workpiece accurate and precise. In addition, such machines can automatically measure targets, record measurement data and obtain GD&T measurements. For this reason, it is worth using the above equipment!